Reliable measurement or estimation of various parameters of a motor vehicle powertrain is essential to proper electronic control of the powertrain components. For example, automatic shifting of a multi-gear ratio power transmission requires a reliable measure or estimation of the transmission output speed. While the essential parameters are often measured with dedicated sensors, the control must include provisions for operation of the powertrain components in the event of sensor malfunctions. For example, if a transmission output speed sensor malfunctions during operation of the vehicle, the powertrain controller can respond by reverting to a “limp-home” control mode in which the transmission only operates in a specified gear ratio. Alternatively, it may be possible for the powertrain controller to reliably determine a default value for the transmission output speed based on other sensor data and provide normal or near-normal control of the transmission based on the default value. For example, the transmission output speed may be calculated based on the vehicle speed if otherwise available, or on the transmission input speed and gear ratio if transmission shifting is not in progress. However, it can be difficult to determine if the vehicle speed signal is reliable, and more than one sensor may be faulty. Accordingly, what is needed is a method of determining a reliable default value of transmission output speed for use in powertrain control in the event of one or more sensor malfunctions.